A woman who moves into an apartment across the hall from two brilliant but socially awkward physicists shows them how little they know about life outside of the laboratory.
Johnny Galecki / Jim Parsons / Kaley Cuoco-Sweeting / Simon Helberg / Kunal Nayyar / Melissa Rauch / Mayim Bialik
Sheldon kicks Amy out to work solo, so she and Leonard bond during a series of science experiments; Howard is replaced by Bert in Footprints on the Moon; Penny proves to be a surprising source of scientific inspiration for Sheldon. Howard and Raj start a
Sheldon and Amy pick their best-man and maid of honor, by doing tricky experiments on their friends. This causes uproar in the group. Meanwhile, Leonard's self-confidence is still badly damaged after reading his brother's Christmas letter.
Sheldon and Amy try to eliminate stress from wedding planning by applying math to the process ending up at city hall. Also, Raj "breaks up" with Howard after realizing his best friend is actually hurting his confidence.
Sheldon tries to teach the guys a lesson after they cut him out of a potentially valuable Bitcoin investment. Also, a seven-year-old video reveals a secret about Leonard and Penny's relationship.
Howard and Leonard become upset after they discover that Sheldon has been working with the military without telling them; Bernadette asks Raj to investigate her suspicions that Ruchi is attempting to take her job away from her.
Sheldon and Wil Wheaton both go head-to-head for the role of the new Professor Proton, Meanwhile Penny takes care of Halley when Bernadette and Howard are on bed rest.
Amy and Howard work in the lab together causing Sheldon, Bernadette and Raj to feel jealous and lonely. Penny and Leonard use a parenting book to control Sheldon's behavior.
Leonard is devastated by the new friendly relation developing between his mother and Penny. Meanwhile Sheldon takes a detour with Raj and Howard to explore the old belongings of Howard's Science Models.
Sheldon and Amy struggle with picking a wedding date. Amy wants it near a cliff which Sheldon doesn't want. Meanwhile Stuart and Raj fight over one of Bernadette's co-workers for a date.
Leonard angers the University and the Physics Community when he gives an embarrassing radio interview. Bernadette and Amy talk about their success. After the embarrassing radio interview Penny comforts her husband.
Amy has trouble deciding whether or not she should marry Sheldon. Meanwhile, Bernadette confides in Penny when she finds out some huge news.
The gang is concerned when Sheldon's former admirer, Dr. Ramona Nowitzki, resurfaces while Amy is away at Princeton.
After celebrating the completion of the top-secret Air Force project, Leonard, Sheldon and Wolowitz get an unwelcome surprise; Amy receives an offer for a summer position as a visiting researcher.
Howard and Bernadette have a hard time leaving Halley in daycare when Bernadette returns to work. Also, Bert introduces the gang to his new girlfriend, Rebecca.
Sheldon reaches his limit when he works on projects with Leonard, Wolowitz and Amy in one day.
Leonard, Penny and Raj adjust to their new living arrangements; Sheldon expresses an interest in Amy's work.
When the gang offers a place for Raj to live, it creates unexpected conflict among everyone and an argument between Amy and Leonard's mother.
The guys' annual trip to Comic Con is in doubt when Raj can't afford to go.
Amy discovers everyone at the university knows about Sheldon's and her romantic encounters. Raj's contemplates trying to live without financial support from his father.
Leonard and Howard's attempt to keep Sheldon away from their Air Force project by sending him on a train trip backfires. Penny and Amy take Bernadette for her first night out since having the baby.
Penny's brother comes to town and Leonard tries to help her land him a job interview. Sheldon learns of a machine that will help him understand human emotions. Raj reunites some of his old girlfriends to figure out why he is still single.
Leonard and Penny ask for Sheldon's and Amy's help on putting the romance back in their marriage. Howard and Raj go to extreme lengths to stop the floor in Halley's bedroom from making noise.
After spending the holidays apart, the gang gets together to hear about Sheldon's and Amy's trip to his mother's, Leonard's and Penny's Christmas tree adventure, and Howard's and Bernadette's first days as parents.
Amy's desire to have an intimate night with Sheldon on her birthday is interrupted by Bernadette going into labor.
Dividing up their possessions becomes an all-out war between Leonard and Sheldon. Stuart asks to live with Howard and Bernadette and tries to supplant Raj's efforts to help with the new baby.
Sheldon can't contain his jealousy when Bert receives a large Geology grant. A new toy Howard buys faces judgment by everyone.
A proposition by Sheldon shocks Amy. Raj takes an interest in someone at the university.
Amy neglects to tell Sheldon her apartment has been repaired. Leonard and Penny discuss what is to be done with Sheldon's room, but Leonard becomes more upset that Penny's been storing away his collectibles.
Sheldon and Amy host a brunch. Penny is surprised by her popularity when her movie, Serial Apeist, is featured at a Van Nuys comic book convention.
Sheldon and Amy go through a period of adjustment with their new living arrangement. Howard and Bernadette are forced to vacation at home when she falls ill and keep it a secret, unaware Raj and Stuart planned to use their hot tub.
When Amy must vacate her apartment for repairs, living arrangements are made with Leonard, Sheldon, and Penny. Howard and Bernadette don't want to know the sex of their baby, but Raj finds out.
Bernadette confides to Raj she isn't sure if she's ready to be a mother. Penny accompanies Amy to a university mixer, and Sheldon goes to unusual extremes to assist Howard with his military project.
Howard is visited by an Air Force captain regarding his invention. Penny tells her co-workers Bernadette is pregnant, not knowing Bernadette was trying to keep it a secret.
As Leonard and Penny attempt to hold another wedding ceremony everyone can attend, they have to deal with Penny's family arriving and Leonard's father spending the night with Sheldon's mother.
Leonard and Penny try to have a wedding ceremony everyone can attend, but it creates serious conflict among Sheldon's mother and Leonard's divorced parents. Howard's invention draws the attention of the Air Force.
Penny has trouble when Leonard's mother comes to town. Sheldon hires Stuart to hang out with Amy while he goes to a movie screening.
Sheldon and Bernadette spend a night together while the rest of the gang goes to a wine tasting party, but things get uncomfortable for Raj when Zack also attends and takes an interest in Claire.
A minor argument between Sheldon and Leonard escalates quickly and drags the rest of the gang into it during a "Game of Thrones" watching party..
Secrets are revealed when Amy and Sheldon join Penny and Leonard on a weekend cabin trip. Raj is more excited about Howard's and Bernadette's impending parenthood than they are.
Amy is shocked by a revelation from Sheldon when she buys him a new laptop. Raj rats on Leonard and Howard to Penny and Bernadette.
Sheldon works up a contract to split their portion of their guidance system invention royalties, but Bernadette does not like how it's done. Raj must again decide between Emily and Claire when they both contact him.
Bernadette has some news to share, but the reactions are not what is expected.
Sheldon and Amy host a Valentine's Day episode of "Fun with Flags", Leonard and Penny grapple with getting older, Howard and Bernadette find a surprise in their hot tub and Raj is torn between Emily and Claire.
Meemaw meets Amy, but it isn't all roses. Meemaw seems to have an axe to grind with her. Sheldon comes to Amy's defense, but not before Meemaw lets a little secret of his slip.
After being forced to deal with a sick Sheldon, the rest of the gang try to treat themselves to a Sheldon-free weekend.
Leonard visits a psychiatrist so Penny can make a sales call on her and they learn some things about their interactions and marital issues. Stuart moves out of the Wolowitz House. Sheldon works with Raj looking for extra-solar planets.
On Thanksgiving, Sheldon and Amy try to spend time together as friends and go to an aquarium, Howard reluctantly agrees to volunteer at the soup kitchen with Bernadette, Raj and Emily, and ...
Sheldon recruits Howard and Raj in order to find him a new girlfriend, while Bernadette, Penny and Leonard spy on Amy's mystery date.
Wil Wheaton interviews Sheldon for a Star Trek documentary where he reveals the ring and his plan to propose to Amy. Bernadette wants to start renovating the house and the topic of starting a family together with Howard is brought up.
Sheldon and Leonard find out that other scientists are trying to steal their Superfluid Helium idea. The gang try and find a new boyfriend for Amy on a new mobile dating app.
The guys want to learn to be better at sports so Barry Kripke teaches them fencing. Meanwhile the girls help Stuart try to get more women to come to the comic book store. Amy gets asked out...
Leonard and Penny finally move in together leaving Sheldon very upset with this big change in his life and wishing he could go back to a time when he had no emotions in 2003 before he met ...
The guys' science skills are put to the test when they get a flat tire during Leonard's bachelor party weekend in Mexico. Meanwhile, the girls force Penny to finally tell her family that she eloped with Leonard.
Leonard confronts the woman he kissed on the North Sea in an attempt to set Penny's mind at ease. Meanwhile, Sheldon films a special episode of "Fun with Flags" after his breakup with Amy, ...
Penny struggles with Leonard's confession that he kissed another woman as they prepare to get married in Las Vegas. Meanwhile, Sheldon doesn't know what to do after Amy pushes pause on their relationship.
As Sheldon presses Leonard and Penny to set a wedding date, he encounters major changes in his relationship with Amy.
Personalities collide when Leonard and Sheldon's mothers finally meet. Meanwhile, Howard finally decides to "man up" and do his fair share of the housework - but not without a little help from his friends.
When Leonard's flight to his alma mater is canceled, he uses any and all means to deliver the speech he was asked to give. Raj's father threatens to cut off his money, and Sheldon and Howard try to fix a helicopter drone.
When Raj is asked to create a message on the off chance a NASA mission discovers alien life, the guys fight over what he should do, while Penny contemplates returning to acting.
When Sheldon and Amy build a blanket fort, they come to major crossroads in their relationship. Meanwhile, Howard meets the half-brother he never knew he had, and Penny is invited to appear on Wil Wheaton's podcast.
When Leonard and Sheldon are invited to speak at UC Berkeley, they take a detour to try and meet one of their idols. Meanwhile, while cleaning out Mrs. Wolowitz's house, a ping pong battle ...
Tension arises when a magazine fails to mention Leonard in an article about the paper he co-wrote with Sheldon, and a blackout at Mrs. Wolowitz's home leads to a final family dinner in her honor.
A trip to the pet store ends in a fight after Amy learns Sheldon has applied for a one-way mission to colonize Mars. Meanwhile, Leonard surprises Penny with a very "adult" purchase, and Raj gets into trouble when he snoops around Emily's apartment.
Sheldon and Penny agree to participate in an unusual experiment. Leonard, Amy, Raj and Emily spend the evening trying to escape a room with a "zombie". Howard and Bernadette run into trouble at the airport after returning from Mrs. Wolowitz's funeral.
Howard receives some shocking news as everyone prepares for the re-opening of the comic book store. Meanwhile, Penny tries to teach Sheldon how to "let it go," but can't follow her own advice when she learns something infuriating about Amy.
Leonard and Sheldon defend themselves against criticism from an online bully, while the girls confront embarrassing moments from their pasts.
When Sheldon becomes stuck in a rut with his work, the gang try to help him by raising his stress levels. Howard teases Raj with a game called "Emily or Cinnamon."
Raj worries a probe he designed will not work in outer space, while Leonard and Sheldon go shopping with Penny and Amy.
Amy hosts a traditional Victorian Christmas Eve dinner, Raj's father comes to visit, and the guys try to shoo out a bird they let into the university clean room.
While Sheldon retires "Fun With Flags", Leonard, Howard and Raj look for something significant in a dead professor's research, and Bernadette finds out what her colleagues really think of her.
Sheldon panics when Leonard announces he has to go into the hospital for surgery. Raj discovers his parents have divorced.
The girls decide to re-enact their high school proms with the guys. While Sheldon feels under pressure to commit to his relationship with Amy, Howard worries about who Stuart will bring as his date.
The guys befriend one of Penny's clients when he shows up at her apartment. Meanwhile, Bernadette gets upset when Amy torpedoes an article about her for a local magazine.
Sheldon and Raj test whether they could survive a dark-matter research expedition in a salt mine by simulating the conditions in a steam tunnel. Meanwhile, Leonard and Penny seek advice from Howard and Bernadette when they argue about money.
The girls head to Vegas, while the guys try to invent something cool, but only come up with new ways to procrastinate.
Raj's honesty about his past comes back to haunt him after Emily takes a dislike to Penny. Meanwhile, the guys consider investing in the comic book store.
Howard becomes nervous after NASA asks him to throw out the first pitch at a Los Angeles Angels game. Meanwhile, Leonard and Penny try to prove Sheldon wrong when he says that he and Amy have a 'superior' relationship.
Sheldon is forced to teach a class as a Junior Professor. Meanwhile, the tension between Penny and Bernadette gives Amy a chance to play both sides.
Sheldon's trip comes to an end when he is robbed, Penny interviews for a job at Bernadette's company, and Howard is weirded out by his mother's relationship with Stuart.
Sheldon gets upset about his career, the destruction of the comic book store and Leonard and Penny's future living arrangements. Howard and Bernadette struggle to keep hold of a caregiver for Mrs. Wolowitz.
Penny gets fired from her movie and takes a serious look at her life. Raj sees Emily on a date with another man at the movies. Howard and Bernadette have to take care of his mother after she breaks her leg.
As the gang celebrate Star Wars Day, Sheldon has difficulty dealing with the recent death of Arthur "Professor Proton" Jeffries.
When Sheldon tries to be spontaneous, it leads to unexpected friction between the girls. Meanwhile, Raj seeks Howard's help in preparing for a date with Emily.
Penny tries to help Sheldon when he reconsiders his "relationship" with String Theory. During a dinner with Howard and Bernadette, Raj discovers Howard once embarrassed himself on a date with Emily.
While Raj is wracked with guilt when he tries to date Lucy and Emily at the same time, Penny wrestles with whether to take a role in a cheesy movie and Sheldon is torn trying to choose between two gaming systems.
When Sheldon visits his mother in Houston, he is forced to confront a new reality. Back in Pasadena, the rest of the gang reluctantly take part in Raj's Murder Mystery party.
After arguing with Sheldon, Howard tries to make amends by taking him to NASA headquarters in Houston. Penny has second thoughts about quitting her waitressing job when her car dies. Amy gets involved in trying to find Raj a date.
Leonard's purchase of a dining room table causes Sheldon to reevaluate the changes in his life. Meanwhile, Howard is offered a chance to go back to space and Bernadette struggles with whether or not to encourage him.
Sheldon and Amy take a train ride to Napa Valley with Howard and Bernadette on Valentine's Day. Leonard and Penny take Raj's dog to the vet when she accidentally eats some chocolate.
When the guys can't get hold of any Comic-Con tickets, Sheldon tries to hold his own convention and winds up spending a wild night with James Earl Jones. The girls go to a hotel for tea and talk about maturity.
Sheldon tries to relax after he is forced to take a vacation. Leonard struggles to be supportive of Penny after she quits her job. Amy receives unwanted attention from a co-worker. Bernadette seeks Stuart's help in replacing one of Howard's comic books.
After Leonard tries to help Penny following a failed role on a TV show, she makes an important decision. Meanwhile, Raj and Stuart try to practice their social skills, and Sheldon navigates the intricacies of being funny.
While Sheldon is at home in Texas, the rest of the gang decorate for Christmas and think about how much he has affected their lives.
Leonard has to deal with the fallout of disproving Sheldon's new scientific discovery. Raj spends a week at Howard and Bernadette's apartment.
The gang spend Thanksgiving at Mrs. Wolowitz's house. Howard tries to bond with his father-in-law, and a mistake from Penny's past comes back to haunt her.
Sheldon demands Leonard make up for a mistake from their past. Penny confronts Raj's ex-girlfriend, Lucy.
Sheldon seeks payback when Arthur "Professor Proton" Jeffries asks Leonard for help on a project instead of him. Raj gets upset at Howard for crashing his night with the girls.
Howard tries to do something special to celebrate the anniversary of his first date with Bernadette. Sheldon is conflicted when he makes a major scientific breakthrough but some details have to be corrected.
After a fight with Bernadette, Howard ends up on Raj's couch. Sheldon and Amy clash when Amy takes a job at the university.
Sheldon gets mad at Amy for ruining one of his favorite movies, Raj and Stuart try to make their online dating profiles more appealing, and Leonard tries to play the sympathy card with Penny.
The gang get competitive when they are forced to take part in Raj's scavenger hunt.
Sheldon feels betrayed when Leonard returns from the North Sea and wants to spend his time with Penny instead of him. Howard's latest foray into helping his mother has some disastrous side effects.
Sheldon and Penny bond over missing Leonard. Howard tries to help Raj get over his breakup with Lucy. Amy and Bernadette take a trip together to a biology convention.
Penny is thrown for a loop when Leonard is offered a job overseas, but the news is an even bigger shock to Sheldon. Raj and Lucy's relationship comes to a crossroad.
A date between Raj and Lucy takes both of them out of their comfort zones. After a trip to Las Vegas goes awry, the girls join in on a Dungeons and Dragons game, which causes Amy and Sheldon to re-evaluate their relationship.
Bernadette and Howard offer to watch Raj's dog and live to regret it. Leonard and Sheldon hire their favorite childhood star to make an appearance.
Amy tries to help a depressed Sheldon find closure when one of his favorite TV shows is canceled. Raj becomes paranoid when he finds Lucy's blog.
Leonard, Sheldon and Raj battle for tenure at the university and drag Penny and Amy into the fight.
Howard becomes upset when Sheldon finds a letter from his estranged father whilst cleaning out his and Bernadette's closet.
Leonard, Sheldon and Howard give lectures on science to junior high students. The girls ditch work to go to Disneyland. Raj devises a date for Lucy that fixes their social anxiety problems.
Raj falls apart after his date sneaks out on him, and Sheldon reluctantly returns the favor after Penny helps him out with his "Fun With Flags" program.
Sheldon gets Alex to buy a Valentine's Day gift for Amy. Leonard and Penny and Howard and Bernadette have a disastrous dinner together. Raj and Stuart host a "Lonely People" party at the comic book store.
Leonard moves in with Penny after a fight with Sheldon, which causes Amy to want to move in with Sheldon. Meanwhile, Raj has a frightening experience with Howard's mother.
Sheldon is forced to work with Kripke on a project and must concoct an elaborate lie when he can't hold up his end of the work. Howard and Raj get superhero figurines made of themselves.
The guys are stranded in the middle of nowhere when Leonard's car is stolen on the way to the Bakersfield Comic-Con. Back home, the girls try to figure out why the guys are so fascinated with comic books.
When Penny finds out Alex has a crush on Leonard, Sheldon gets himself and the rest of the guys in trouble with Human Resources whilst trying to solve the problem.
The girls take Raj out to a nightclub to find him a date. A game of Dungeons and Dragons rekindle old memories of Santa for Sheldon.
Howard tries to bond with Bernadette's father during a fishing trip. When Sheldon is obligated to take care of a sick Amy, she finds ways to take advantage of the situation.
The gang are drawn into a fight between Howard and Sheldon over Sheldon's vacant parking spot at the university.
Leonard feels threatened after hearing about Penny's study partner at school, while Howard and Raj become determined to find out one of Sheldon's secrets.
Howard makes a decision whether or not to move out of his mother's house, while Sheldon is caught in the middle of an argument between Amy and Wil Wheaton.
Penny enrolls in a college class and keeps it a secret from Leonard. Sheldon causes conflict with Dr. Stephen Hawking over an online game.
While Raj plans Stuart's Halloween party at the comic book store, Howard annoys the rest of the gang by constantly talking about space, and Penny tries to take more of an interest in Leonard's work.
Howard is offended when his return from space is met with little fanfare. Penny and Amy take on Sheldon and Leonard during game night.
When Sheldon hires a female grad student to assist him, a jealous Amy asks Penny to help her spy on them. On the space station, Howard begins behaving erratically.
Penny continues to have doubts about her relationship with Leonard. Meanwhile, Howard becomes the target of practical jokes on the space station.
Raj crashes Sheldon and Amy's second anniversary and interferes in Leonard and Penny's relationship. Howard gets caught in the middle of an argument between Bernadette and his mother.
As Howard prepares for his mission into space, flashbacks show the recent events leading up to his and Bernadette's wedding.
Howard faces double trouble when NASA moves up his launch and he has to meet Bernadette's father. Leonard inadvertently jeopardises his relationship with Penny.
The guys throw Howard a bachelor party, and some embarrassing details about his life get posted on the Internet.
Sheldon uses any and all means to meet his hero, Dr. Stephen Hawking.
Raj gets quite a surprise when he asks his parents to set him up on a date. Penny buys Leonard and Sheldon a pair of Star Trek keepsake collectibles.
Sheldon has to choose between joining Amy at her aunt's birthday party or playing videogames all weekend with the guys.
Sheldon wreaks havoc when he tries to get a haircut from his regular barber, who is ill. Howard runs into a few problems with astronaut training.
Sheldon battles Kripke for a coveted office at the university. Penny receives a very unusual gift from Amy.
When the university forces Sheldon to use vacation days, he decides to hang out in Amy's lab. Howard becomes upset when he hears Bernadette wants him to sign a prenuptial agreement.
Leonard signs out of his roommate agreement, reducing himself and Sheldon to mere acquaintances, while Howard tries to think of a cool astronaut nickname for himself.
Leonard and Penny develop a system to ensure their dates go smoothly. Raj finally finds a woman he can talk to - the virtual assistant on his mobile phone.
Leonard and Penny contemplate reviving their romantic relationship.
Penny helps Sheldon work on his relationship with Amy when he trivializes one of her accomplishments. Howard is irked by Bernadette's dislike of children.
Leonard confronts the man who bullied him in high school, while Penny realizes she did her fair share of tormenting in the past.
When Stuart asks Amy on a date, Sheldon considers having a more serious relationship with her. Meanwhile, the guys become obsessed with a new card game involving Wild West heroes and witches.
Sheldon tries to overcome his fear of birds, while Leonard and Penny attempt to hang out together strictly as friends.
Amy is hurt when Penny and Bernadette go bridesmaid dress shopping without her, so she turns to a reluctant Sheldon for comfort.
Sheldon tries to get into the spirit of Halloween by trying to scare the guys after they prank him. Leonard seeks Penny's advice when an attractive comic book artist takes an interest in him.
Sheldon gets upset when his mother comes to visit but refuses to cook for him and attend physics lectures.
Sheldon takes offense when Leonard wants to attend a party with Wil Wheaton, while Bernadette tries to block Howard's opportunity to go into outer space.
Penny sets up Raj with a deaf friend, not knowing that she's actually a gold digger, and Sheldon decides to let his minor decisions rest on the roll of the dice.
Leonard accompanies Amy to a colleague's wedding. Howard tries to convince Bernadette to live with his mother after they get married.
Amy gets inadvertently caught in the middle of a fight between Sheldon and Penny over a chair. Leonard tries to add some spice to his long-distance relationship with Priya.
The gang deal with the fallout from Penny and Raj's night together. Amy counsels Penny when she considers moving back to Nebraska, Bernadette confronts Raj about his crush on her, and Sheldon takes charge of the paintball team.
Raj moves in with Sheldon after hearing Leonard and Priya engage in a Star Trek fantasy in his apartment. Bernadette receives her doctorate.
Howard believes that his mom suffered a heart attack because he told her that he's marrying Bernadette, Sheldon freaks out over germs, and Penny and Priya bond.
Sheldon invents a three-player chess game, Penny and Amy use Bernadette to spy on Leonard and Priya, and Raj tries a new drug to treat his social anxiety.
When Priya finds a loophole in the roommate agreement to throw Sheldon out for the night, Penny, Amy and Bernadette take him to a nightclub.
Sheldon and Amy secretly experiment on the gang by spreading rumors when they hear that Bernadette is thinking about breaking up with Howard.
The guys go all out to help Sheldon find the person responsible for hacking his online gaming account, while the girls' hatred for Priya increases.
Priya forces Leonard to choose between a relationship with her or a friendship with Penny. A magic trick by Howard has Sheldon fooled.
Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group.
Bernadette makes Howard choose between herself and his mother, while Penny and Raj become upset when Leonard and Priya start dating.
To secure needed funding for the university, Sheldon convinces Leonard to get friendly with a wealthy benefactor.
To improve his skills as an instructor, Sheldon asks Penny to give him acting lessons. Meanwhile, Raj begins having impure fantasies about Bernadette.
Chaos ensues when the gang spend the weekend together at a science convention.
Leonard has an idea for a new smartphone app and asks the guys for help, however Sheldon tries to take over the project, and Leonard fires him.
The guys enter a costume contest as members of the Justice League and convince Penny and Zack to join them.
Amy finds she has sexual feelings for Penny's ex-boyfriend Zack, while Raj and Howard try to prove who would be the better superhero.
Penny asks Leonard to pose as her boyfriend when her father visits. Howard, Raj and Bernadette have an awkward encounter when they spend the evening together.
The guys camp out all night waiting to see an extended cut of Indiana Jones, while Penny and Bernadette invite Amy to her first slumber party.
An FBI agent interviews the guys when Howard needs to obtain a security clearance.
Leonard tries to keep everyone from finding out that he spent the night with Raj's sister, but Sheldon finds out and concocts an elaborate story to help him keep the secret from Raj.
Sheldon is scared that he is in a relationship when Amy asks him to meet her mother, and Leonard asks Howard and Bernadette to set him up because he is the only person in the group without a girlfriend.
Howard tries to get Penny's help in winning back Bernadette, but first has to confess the embarrassing reason why they broke up in the first place. Raj and Sheldon clash over a desk.
Following his 'breakup' with Amy, the gang become concerned with Sheldon's obsessive search for non-human companionship and call his mother to intervene.
When Sheldon calculates the technology to download his consciousness into a robot won't be invented soon enough, he desperately tries to find a way to increase his lifespan.
Penny ends up tagging along on Sheldon's first ever date with Amy, while Howard finds an unusual use for a robotic arm.
Penny has a night of angry, drunken sex with Leonard, much to his confusion. Meanwhile, Howard and Raj search for Sheldon's perfect match online.
Leonard tells Penny about how he and Sheldon became roommates and what happened to the elevator.
Chaos ensues when a nymphomaniac physicist visiting Sheldon hooks up with Leonard.
Sheldon is forced to go behind Leonard's back to spend time with Penny following their breakup.
Sheldon takes on Wil Wheaton in a game of bowling. Leonard and Penny's relationship comes to a crossroad.
Sheldon seeks help from the rest of the gang in overcoming his fear of public speaking when he receives an award.
The guys' friendship is threatened when they find a ring from the "Lord of the Rings" trilogy.
Sheldon misses an opportunity to get Stan Lee's autograph because he has to make an appearance in traffic court.
When Leonard is offered a trip to Switzerland on Valentine's Day to visit the CERN laboratory, Sheldon uses any and all means to be his guest instead of Penny.
Sheldon wreaks havoc when he is stumped by a difficult physics question.
When their apartment is robbed, Leonard and Sheldon turn to Howard and Raj to create a state-of-the-art security system.
Raj talks Sheldon into being his wingman at a university mixer. Leonard has trouble dealing with Penny consulting a psychic.
Leonard's mother makes a visit to his dismay and Sheldon's delight.
Sheldon attempts to help Penny understand physics to impress Leonard, while Howard becomes jealous when Bernadette takes an interest in Leonard's research.
Sheldon plans revenge after Kripke humiliates him on the radio. Howard develops doubts about his relationship with Bernadette.
Penny is forced to turn to Sheldon for help when she dislocates her shoulder in the bathtub. Meanwhile, the rest of the guys accidentally get stoned in the desert while waiting for a meteor shower.
Sheldon is caught in the middle when Leonard and Penny argue about Penny's friend staying at her apartment.
Leonard tries to learn about football to fit in with Penny's friends. Meanwhile, a loss in a kite war threatens to end Raj and Howard's friendship.
Leonard is forced to ask Penny to set Howard up with one of her friends, while Sheldon seeks payback against sci-fi star Wil Wheaton.
When Raj has to find a new job or be sent back to India, Sheldon's solution leaves Leonard and Penny dealing with Howard as the third wheel.
Sheldon attempts to covertly alter Penny's habits, while Howard and Raj try their luck meeting women at a Goth-style club.
Leonard and Penny's first night together goes awkwardly. Sheldon and Howard wager on the species of a cricket.
After returning from the North Pole, a conflict among the guys sends Sheldon back to Texas and interferes with Leonard's reunion with Penny.
Penny gets upset when she finds out Leonard and the guys plan to work at the North Pole for the summer.
Penny's second date with Stuart has significant implications for Leonard. Howard enlists the guys' help with one of his NASA projects.
After being dumped by Leslie, Howard heads to Las Vegas with Leonard and Raj. Meanwhile, Penny is forced to take in Sheldon after he locks himself out of his apartment.
Leonard is so upset when a friend from the comic book store begins dating Penny that he convinces Howard and Raj to take him to a bar so that he can pick up a woman of his own.
Penny becomes jealous when a gorgeous new neighbor attracts the guys' attention.
Sheldon's assistance with Penny's home-based business leads to disaster.
On a train to San Francisco, Howard, Raj and Leonard fight for the affections of sci-fi star Summer Glau, while Sheldon is forced to turn to Penny for help when he leaves his flash drive behind.
Howard hooks up with Leslie, while Penny accidentally ruins Sheldon's spot on the couch.
Sheldon hits it off with Leonard's psychoanalyst mother, but she drives Leonard and Penny to the bottle.
Leonard and Sheldon try to help out Penny when she can't afford to pay her rent.
Sheldon tries to make friends with Kripke in the hope that it will get him access to a coveted computer in the lab.
Howard becomes depressed after Penny makes a cruel statement about his love life, while the rest of the guys prepare for a grudge killer robot duel against an obnoxious colleague at the university.
Leonard is disappointed when Penny develops an interest in an attractive physicist at the university. Meanwhile, Sheldon struggles to find an appropriate Christmas gift for Penny.
Leonard tries to slow down his relationship with Stephanie when he realizes that they are living together, while Sheldon becomes convinced that he has an untraceable disease.
Sheldon believes that Leonard will wreck his relationship with Stephanie, so he decides to take matters into his own hands.
Leonard and Howard's friendship is jeopardised when Leonard steals a girl from Howard.
Penny plans revenge when Sheldon bans her from the guys' apartment, while Howard and Raj use the latest technology to try to find the house being used in "America's Next Top Model."
Sheldon is flattered when a graduate student takes a shine to him, however her extreme devotion becomes too much to handle.
Sheldon annoys the gang when Leonard can't drive him to and from work, so they try to teach him how to drive.
The guys struggle to cope with Raj's arrogance after he is featured in a People magazine article.
Sheldon introduces Penny to online gaming, however she refuses to quit after becoming addicted.
Sheldon is annoyed when Leonard turns to Leslie for comfort after seeing Penny with another guy.
Leonard becomes concerned when his date with Penny ends abruptly and she starts blowing him off. When told the truth, Sheldon would rather move out than keep Penny's reasons a secret from Leonard.
After a bad breakup, Penny finally agrees to go out on a date with Leonard, however they both develop doubts and turn to Sheldon for advice.
When Penny learns that Leonard has never had a birthday party, she and the rest of the guys plan a surprise party for him.
Leonard, Howard and Raj fight over Sheldon's twin sister when she arrives in town. Raj takes part in a drug trial to suppress his selective mutism.
Penny gets mad at the guys when their full scale model of a time machine causes her to miss work, which prompts Leonard to give up all of his nerd memorabilia.
Sheldon becomes so intent on demonstrating his intellectual superiority over the other guys that they kick him off the Physics Bowl team and replace him with his nemesis, Leslie.
Sheldon decides to give up his work and focus on other tasks when a 15-year-old prodigy joins the lab, so the other guys come up with a plan to get rid of him.
When Sheldon gets sick, Leonard, Howard and Raj go AWOL, leaving a reluctant Penny to deal with him.
Leonard lies to Penny so that he and Sheldon can get out of watching her perform. However, Sheldon believes that the lie has too many loose ends, so he comes up with a new, unnecessarily complex one to replace it.
Leonard and Sheldon's friendship is put to the test when Leonard wants to present a paper they co-authored at a physics convention, but Sheldon doesn't.
When Raj's parents set him up on a blind date, he finds he can talk to women with the aid of alcohol.
When Howard hooks up with Penny's old friend, his absence in the gang causes problems for the rest of the guys.
The guys are invited to Penny's Halloween party, where Leonard has yet another run-in with Penny's ex-boyfriend Kurt.
Leslie seduces Leonard, but afterwards tells him that she is only interested in a one-night stand.
Sheldon's mother is called to intervene when he delves into numerous obsessions after being fired for being disrespectful to his new boss.
Leonard gets upset when he discovers that Penny is seeing a new guy, so he tries to trick her into going on a date with him.
Penny is furious with Leonard and Sheldon when they sneak into her apartment and clean it while she is sleeping.
A pair of socially awkward theoretical physicists meet their new neighbor Penny, who is their polar opposite.